1. Field of the Invention
The present invention relates to a color temperature conversion system and method thereof, and more particularly, to a color temperature conversion system for converting a color in a color display system and a method using the same. The present application is based on Korean Patent Application No. 2001-63418, filed Oct. 15, 2001, which is incorporated herein by reference.
2. Description of the Prior Art
When one looks at an object, the overall hue perceived by the viewer varies depending on characteristics of the illumination. For example, under an incandescent lamp, an object appears reddish, while in daylight, the same object appears more bluish than under the incandescent lamp. That means, the higher color temperature results in bluish colors, and the lower color temperature results in reddish colors. Since the color is closely related to the color temperature, the color temperature needs to be changed in order to change the color.
Color display systems are widely used in many devices for visually transmitting information to the viewer. Such devices include, for example, TVs, DTVs, thin film transistor (TFT) monitors, color printers, digital cameras, projectors, and mobile phones. Due to visual characteristics of the color display system, a correlated color temperature needs to be calculated accurately. The correlated color temperature of a light source which is measured in Kelvin units (K) is the temperature of a blackbody when the color of the light source is the same. In other words, the correlated color temperature is a wavelength of the light source represented by the Kelvin scale, a standard comparison measuring value.
Color models are used to classify colors, and also qualify colors in accordance with attributes like hue, saturation, chroma, lightness, and brightness. Also, the color models are used for matching, and at the same time the valuable resources for the subjects that have to handle color images on the video or the Web. The color models include a RGB model, a HSB/HSL model, a Munsell color system, and a CIE color model. The CIE color model is determined by the International Commission on Illumination, the organization for determining lighting standards. The CIE color model includes CIEXYZ, CIELUV, and CIELAB color models. The CIEXYZ color model uses positive tristimulus values, indicated as XYZ to express colors. The CIEXYZ color model uses a chroma diagram.
Conventionally, the color temperature is calculated by the Robertson algorithm that uses X and Y axes of the CIEXYZ color model. FIG. 1 is a flowchart for detecting the color temperature by using the Robertson algorithm, and FIG. 2 is a view illustrating the color temperature detecting method of FIG. 1. With reference to FIGS. 1 and 2, the color temperature detecting method using the Robertson algorithm will be described below.
The description of the conventional method for calculating color temperature is based on “Color Science: Concepts and Methods, Quantitative Data and Formulae, 2nd edition, pp. 225–229 & p. 828, 1982” by Gunter Wyszecki and W. S. Stiles. According to the conventional method for calculating color temperature, the color temperature of the light source is obtained by selecting an isotemperature line most adjacent to two-dimensional chroma coordinates corresponding to the light source. The isotemperature line is the line representing values of color temperature with respect to the selected light source. Accordingly, the two-dimensional chroma coordinates of the CIE 1931 diagram calculated from the RGB chroma coordinates of the input image, in other words, the XY chroma coordinates (xs, ys) are inputted (step S110).
Then the XY chroma coordinates (xs, ys) are converted into the two-dimensional chroma coordinates (hereinafter called UV chroma coordinates) (Us, Vs) of the CIE 1960 UCS diagram (step S120). The UV chroma coordinates (Us, Vs) are calculated by the following mathematical Equation 1:                                           U            s                    =                                    4              ⁢                              x                s                                                                                      -                  2                                ⁢                                  x                  s                                            +                              12                ⁢                                  y                  s                                            +              3                                      ,                              V            s                    =                                    6              ⁢                              y                s                                                                                      -                  2                                ⁢                                  x                  s                                            +                              12                ⁢                                  y                  s                                            +              3                                                          [                  Equation          ⁢                                          ⁢          1                ]            
After the UV chroma coordinates values (Us, Vs) are calculated from the UV chroma coordinates of the CIE 1960 UCS diagram, two isotemperature lines that are most adjacent to the UV chroma coordinates (Us, Vs) are selected (step S130).
Next, distances (dj, dj+1) between the selected the UV chroma coordinates (Us, Vs) and the two isotemperature lines are obtained (step S140). The distance dj is calculated by the following mathematical Equation 2:                               d          j                =                                            (                                                V                  s                                -                                  V                  j                                            )                        -                                          t                j                            (                                                U                  s                                -                                                      U                    j                                    )                                                                                        (                              1                +                                  t                  j                  2                                            )                                      1              /              2                                                          [                  Equation          ⁢                                          ⁢          2                ]            
where, the values Us and Vs are UV chroma coordinates with respect to the inputted image.
The values Uj and Vj are UV chroma coordinates on the (j)th isotemperature line that has a slope of tj.
As shown in FIG. 2, since the inputted chroma coordinates (Us, Vs) lie between the two isotemperature lines of A and B, the distance ratio dj/dj+1, calculated by the Equation 2, is always negative.
Next, from the two distance values (dj, dj+1) calculated in S140, a correlated color temperature Tc is calculated (step S150). The correlated color temperature, corresponding to the inputted UV chroma coordinates (Us, Vs) can be obtained by two assumptions. One assumption is that a Planckian Locus (PL) between the two selected color temperatures Tj, Tj+1 can be replaced by a circular arc having a center at an intersection of the two selected isotemperature lines A and B. The other assumption is that the correlated color temperature is a linear function of distance along the circular arc. As a result, the following mathematical Equation is obtained:                               T          c                =                              [                                          1                                  T                  j                                            +                                                                    θ                    j                                                                              θ                      j                                        -                                          θ                                              j                        +                        1                                                                                            ⁢                                  (                                                            1                                              T                                                  j                          +                          1                                                                                      -                                          1                                              T                        j                                                                              )                                                      ]                                -            1                                              [                  Equation          ⁢                                          ⁢          3                ]            
where, θj and θj+1 are angles between the correlated isotemperature lines meeting with the intersection of the isotemperature lines A and B, after passing through the two isotemperature lines Tj, Tj+1 and the UV coordinates (Us, Vs). With respect to the adjacent isotemperature line having small angles θj and θj+1, with the Equations θj/θj+1=sin θj/sin θj+1, a fourth mathematical Equation is obtained as follows:                               T          c                =                              [                                          1                                  T                  j                                            +                                                                    d                    j                                                                              d                      j                                        -                                          d                                              j                        +                        1                                                                                            ⁢                                  (                                                            1                                              T                                                  j                          +                          1                                                                                      -                                          1                                              T                        j                                                                              )                                                      ]                                -            1                                              [                  Equation          ⁢                                          ⁢          4                ]            
As described above, the conventional method for calculating the two-dimensional color temperature is complicated, and also requires a two-dimensional calculator using the UV chroma coordinates to build hardware. Taking the accuracy of the correlated color temperature Tc into account during the establishment of the hardware, a square root calculator is also required to calculate the distance dj. In addition, a comparator, a multiplier, and a divider, etc. are also required to determine a range of the input value. As a result, establishing the conventional method for detecting the two-dimensional color temperature at a hardware level has considerably low practicality and low utilization in terms of size and costs.
In order to overcome the above-mentioned problems, a color temperature conversion system has been suggested. The suggested color temperature conversion system calculates a color temperature with a one-dimensional chroma value, and converts a color temperature of the RGB of an input image into a color temperature of a desired RGB based on the calculated color temperature.
FIG. 3 is a view schematically showing a conventional color temperature conversion system. Referring to FIG. 3, the conventional color temperature conversion system includes an XYZ conversion unit 301, a pixel sort unit 303, a chroma calculate unit 305, a color temperature calculate unit 307, a conversion matrix calculate unit 309, a matrix calculate unit 311, and a target color temperature output unit 313.
The XYZ conversion unit 301 converts an RGB signal of input image data into an XYZ value of CIE coordinate system to detect a perception light source. The pixel sort unit 303 receives the XYZ value of the CIE coordinate system converted by the XYZ conversion unit 301, and eliminates pixels unnecessary for calculation of a light chroma Xs. After the unnecessary pixels are eliminated from the XYZ value of the CIE coordinate system, the XYZ value is transmitted to the chroma calculate unit 305. Based on the one-dimensional lighting chroma Xs inputted from the chroma calculate unit 305, the chroma calculate unit 307 calculates the color temperature of the input image data by the following equation:                                                         1              )                        ⁢                                                  ⁢            If            ⁢                                                  ⁢            4000            ⁢            K                    ≤                      T            c                    <                      7000            ⁢            K                          ,                                  ⁢                                                                              X                  D                                =                                ⁢                                                                            -                      4.067                                        ⁢                                                                  10                        9                                                                    T                        c                        3                                                                              +                                      2.9679                    ⁢                                                                  10                        6                                                                    T                        c                        2                                                                              +                                                                                                                        ⁢                                                      0.09911                    ⁢                                                                  10                        3                                                                    T                        c                                                                              +                  0.244063                                                                                        [                  Equation          ⁢                                          ⁢          5                ]                                                                    2              )                        ⁢                                                  ⁢            If            ⁢                                                  ⁢            7000            ⁢            K                    ≤                      T            c                    <                      25000            ⁢            K                          ,                                  ⁢                                                                              X                  D                                =                                ⁢                                                                            -                      2.0064                                        ⁢                                                                  10                        9                                                                    T                        c                        3                                                                              +                                      1.9018                    ⁢                                                                  10                        6                                                                    T                        c                        3                                                                              +                                                                                                                        ⁢                                                      0.24748                    ⁢                                                                  10                        3                                                                    T                        c                                                                              +                  0.237040                                                                                                    
where Tc is the color temperature of the inputted image data.
As shown in FIG. 4, the color temperature calculated by the one-dimensional chroma coordinate values are positioned on the white line of the CIE coordinate system, while the lighting chroma values of most image are positioned in near to the white point line. Accordingly, the color temperature calculated by the equation 5 merely has a difference with the color temperature detected by using the Robertson algorithm.
The conversion matrix calculate unit 309 calculates the conversion matrix to calculate a target color temperature. The conversion matrix is calculated by the following equation 6, by using the interrelation between the color temperature of the input image data and the target color temperature:[TM]=[(XYZ)MT]−1[(XYZ)ME]  [Equation 6]
where, [TM] is a matrix of the color temperature of the input image, [(XYZ)MT]−1 is a matrix of target color temperatures, and [(XYZ)ME] is a matrix of the input image color temperatures.
The matrix calculate unit 311 multiplies the conversion matrix calculated by the conversion matrix calculate unit 309 by a matrix for a conversion of the XYZ signal in the CIE coordinate system into a RGB value. By doing so, the conversion matrix with respect to the CIE coordinate system is converted into the conversion matrix with respect to the RGB. The conversion matrix, converted by the matrix calculate unit 311, is transmitted to the target color temperature output unit 313. The target color temperature output unit 313 multiplies the conversion matrix received from the matrix calculate unit 311 by the XYZ-converted matrix of the input data, to thereby output an RGB value of the target color temperature, i.e., output the image data of target color temperature. The output image data is expressed by the following equation 7:                               [                                                                      R                  t                                                                                                      G                  t                                                                                                      B                  t                                                              ]                =                                            [              TM              ]                        ′                    ⁡                      [                                                            X                                                                              Y                                                                              Z                                                      ]                                              [                  Equation          ⁢                                          ⁢          7                ]            
where,       [                                        R            t                                                            G            t                                                            B            t                                ]     are RGB elements according to the target color temperature,
[TM]′ is a conversion matrix, and       [                            X                                      Y                                      Z                      ]     is tristimulus according to the input image color temperature.
As described above, considering a problem of hardware size, a general color temperature conversion system usually does not use the Robertson algorithm requiring the use of the lighting chorma values Xs and Ys, but uses the relational Equation between Xs and the color temperature, i.e., equation 5, to obtain the color temperature. The color temperature, obtained as above, is positioned on the white point line of FIG. 4. Since the lighting chorma values are generally positioned near the white point line, there is no considerable difference between the color temperature detected by the Robertson algorithm and the color temperature detected by equation 5.
Using equation 5, however, suffers a drawback of inaccurate detection of color temperature when a monochrome color like green image is inputted, because the image of monochrome color pattern is spaced far beyond the white point line. Also, since the equations used in the color temperature conversion system have a limited range of color temperature of 4,000K˜25,000K, an image of color temperature lower than the limited range has the fixed color temperature of 3,000K in the hardware (equation of 4,000K˜7,000K is used if color temperature is in 3,000K˜4,000K), and accordingly, a reddish image of relatively low color temperature is not detected precisely.
Due to the problems described above, the images being inputted are distorted, and the conventional color temperature conversion system does not convert the color temperature of the input image into a color temperature desired by a user.